In recent years, devices such as facsimile systems and printers have undergone a marked development. In particular, such devices have been modified to utilize a heat-sensitive recording system including the combination of a heat-sensitive recording sheet and a thermal head, said recording sheet being coated with a colorless dye such as Crystal Violet lactone and a phenol compound. For example, the system described in Japanese Pat. Publication No. 14039/70 (British Pat. No. 1,135,540) is widely used in such devices.
This heat-sensitive recording system has many advantages, e.g., since the recording sheet involves first order color formation, no development is needed and a recording apparatus can be simplified. Furthermore, the production costs of the recording sheet and the recording apparatus are low, and there are no noise problems because the recording system is of the nonimpact type. Thus, the system has attained a position as a low-speed recording system in the recording technology.
The heat-sensitive recording system, however, suffers from the serious disadvantage that the recording speed is low compared with other recording systems such as electrostatic recording system. Therefore, it is not employed in high-speed recording.
The major factor preventing high-speed recording with a heat-sensitive recording system is insufficient heat conduction between a thermal head and a heat-sensitive recording sheet coming into contact with the thermal head. Because of the insufficient heat conduction, sufficient recording density cannot be obtained. The thermal head comprising an assembly of dot-like electric resistance heating elements generates heat on application of recording signals and melts the heat-sensitive color forming layer in contact therewith, thereby causing it to form color therein. In order to obtain sharp and high density recording, it is required that dot reproducibility is high. More specifically, the thermal head and the heat-sensitive color forming layer come into contact as closely as possible so that the heat conductivity is achieved efficiently and completely colored dot patterns corresponding to the dot heating elements of the thermal head are formed in the heat-sensitive color forming layer exactly according to high-speed recording signals. At present, however, only several percents of the heat generated from the thermal head is transferred to the heat-sensitive color forming layer. Accordingly, the efficiency of heat conductivity is extremely low.
In order to overcome the problem, several methods of increasing the smoothness of the heat-sensitive color forming layer have been proposed. Increasing smoothness is done in order to bring the thermal head and the heat-sensitive color forming layer into as close a contact as possible.
Japanese Patent Publication No. 20142/77 describes a method in which the heat-sensitive color forming layer is subjected to a surface treatment to the extent that the Bekk smoothness is from 200 to 1,000 seconds. Japanese Patent Application (OPI) No. 15255/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") describes that the heat-sensitive color forming layer subjected to a surface treatment to the extent that the Bekk smoothness is from 200 to 1,000 seconds can respond only to thermal pulses of about 5 to 6 milliseconds. Accordingly, it is necessary in high-speed recording utilizing heat pulses of 1 millisecond or less that the heat-sensitive color forming layer is subjected to a surface treatment to the extent that the Bekk smoothness is at least 1,100 seconds. IF the Bekk smoothness of the heat-sensitive layer is made higher than 1,100 seconds by pressing the heat-sensitive recording paper, color fogging is formed by pressure. However, after the heat-sensitive recording layer is coated on the base paper increasing the smoothness to the Bekk smoothness above 500 seconds, if the Bekk smoothness is increased above 1,100 seconds by adjusting the surface of the heat-sensitive recording paper, the resulting recording paper can give high recording density without increasing fog formation. In Japanese Patent Application (OPI) No. 156086/80 (British Pat. No. 2,051,391), it is described that the surface roughness Ra and gloss of the surface of a heat-sensitive color forming layer are made 1.2 .mu.m or less and 25% or less, respectively.
In accordance with conventional techniques, the smoothness of the heat-sensitive color forming layer has been increased only by calender treatment such as super calendering, machine calendering, and gloss calendering. Such calender treatments are only applied to the base paper, or the base paper and the heat-sensitive recording sheet, or only to the heat-sensitive sheet. With heat-sensitive recording sheets improved in smoothness by the above-described calender treatments, as the recording density is increased by increasing the smoothness, sticking and piling occur more frequently. In practice, therefore, the recording density and the occurrence of such sticking and piling are balanced appropriately by controlling the smoothness to a suitable level. Thus, even if the smoothness is adjusted to any level by the conventional methods, the resulting heat-sensitive recording sheet is not suitable for use in high-speed recording with respect to recording density or record stability.
The sticking phenomenon is created when the thermal head sticks to the heat-sensitive color forming layer, producing a peeling noise and reducing dot reproducibility. The piling phenomenon is created when the heat-melted product (fused material) of the heat-sensitive color forming layer is deposited on the thermal head, reducing the recording density and dot reproducibility. They are both phenomena interfering with stable recording.
Another disadvantage of the calender treatment is that colored fog is formed by pressure, resulting in an increase in density of the background of the recording sheet.
Furthermore, even if the smoothness is increased to, for example, a Bekk smoothness of at least 1,000 seconds by super calendering, the recording density is not always increased. In some cases, the fine unevenness in basis weight is enhanced and the close contact between the heat-sensitive color forming layer and the thermal head is reduced and, therefore, the recording density drops below the maximum value.
When the smoothness is increased by subjecting the base paper or heat-sensitive recording sheet to calender treatments, the thickness of the resulting heat-sensitive recording sheet is reduced, and the density of the heat-sensitive color forming layer and the base paper is increased. This means that the void contents of the heat-sensitive color forming layer and the base paper layer adjacent to the heat-sensitive color forming layer are reduced. Although the heat conductivity of the heat-sensitive color forming layer is slightly improved, it is believed that substances heat-melted in recording are prevented from permeating from the surface of the heat-sensitive color forming layer through the heat-sensitive color forming layer and the base paper layer adjacent thereto into the interior of the heat-sensitive recording sheet. Therefore, the heat-melted substances remain mainly on the surface of the heat-sensitive color forming layer, causing sticking of the heat-sensitive color forming layer and piling on the thermal head.
Increasing the smoothness of the heat-sensitive color forming layer and the recording density by calender treatments inevitably leads to sticking, piling, and colored fog. It is, therefore, difficult to satisfy the both at the same time. At present, therefore, the sticking and piling are reduced and emphasis is placed on the stable recording performance. Accordingly, sufficient recording density cannot be obtained by high-speed recording.